This invention relates to starch-based corrugating adhesives. One usual method of making corrugated board consists of corrugating a strip of paper by means of a corrugated roller, applying an adhesive to the tips of the flutes on one side of that paper and adhering another strip of paper called a "liner", to the flute tips by use of heat and under high pressure.
The resultant product, called a "single face" corrugated board, may be used as is. However, it is usual to make "double face" (also called "double back") board by sending the single face board into a second stage of the corrugating machine called the "double backer" stage. Adhesive is there applied to the opposite flute tips, and a second liner of paper is adhered to that opposite side by the use of heat and under a relatively small amount of pressure. The use of a great amount of pressure in the adherence of the second strip of paper would tend to crush the corrugations. This makes the adhesive problem in the second step quite difficult.
Starch-based corrugating adhesive formulations have been in wide use since the advent of the Stein-Hall technology as disclosed in U.S. Pat. No. 2,051,025, granted Aug. 18, 1936, to J. V. Bauer, and U.S. Pat. No. 2,102,937, granted Dec. 21, 1937, also to J. V. Bauer.
A Stein-Hall type adhesive is a two component aqueous system. One component in this system is generally formed from a cooked, or gelatinized starch material which serves as a carrier phase. The second component or phase is formed from a raw, ungelatinized starch material. The second phase is a latent or potential adhesive phase which is only fully developed after the adhesive has been applied to the tips of the flutes of the corrugated web, the liner has been pressed against the adhesive-coated flutes, and heat and pressure have been applied to cause it to gelatinize and develop structure.
The raw starch material in the adhesive swells and gelatinizes as the newly assembled corrugated board is passed through a hot plate-dryer system that is associated with a corrugating machine. The system also partially dries the corrugated board and sets the adhesive sufficiently so that it can be subjected to subsequent operations, such as trimming, slitting, and sheeting without delamination.
The initial degree of cohesiveness in the bond of the corrugated board is referred to as the green bond strength. This characteristic determines the ability of the newly formed corrugated board to resist the instantaneous high shear forces that are developed during subsequent processing operations, such as the trimming, slitting, and sheeting operations, and is not necessarily an indication of the final bond strength.
After corrugated board has been trimmed, slit, and sheeted, it is stacked and sent to storage. There the adhesive cures to full strength. Until the bond is dry and fully cured, the corrugated board may be delaminated by slowly and firmly pulling the liner away from the corrugated sheet.
Since the middle thirties, one of the major advances in corrugating adhesive technology is that disclosed in U.S. Pat. No. 3,355,307, granted Nov. 28, 1967, to John J. Schoenberger and Raymond P. Citko. That patent discloses a single phase corrugating adhesive referred to as a "no-carrier" system. In this system, partially swollen starch granules are present as a homogeneous phase, suspended in an aqueous, alkaline vehicle. The elimination of the carrier phase permitted substantial operating economies. The no-carrier type corrugating adhesive disclosed and claimed in the Schoenberger-Citko patent is applied and cured in the same manner as the Stein-Hall type adhesive formulations.
A drawback of some no-carrier adhesives, however, is that they may not exhibit a viscosity which will facilitate application of the adhesive to the tips of the flutes during corrugation. The difficulties inherent in obtaining a precise, partial degree of swelling make close control over this viscosity difficult. As a result, uniform application of the adhesive to the flute tips may become quite difficult.
The foregoing drawbacks are particularly evident during high speed corrugation such as that obtainable by the recent advance set forth in U.S. patent application Ser. No. 795,263 filed May 9, 1977 now abandoned of Gary H. Klein and Merle J. Mentzer. At higher speeds, the tolerance for adhesives having poor rheological characteristics is correspondingly reduced.
That patent application, the disclosure of which is incorporated herein by reference, includes disclosure of a no-carrier adhesive, an essential component of which is a starch material formed by saponification of starch ester having at least about 30% greater area under an Instron force-time heating curve and at least about 30% greater area under an Instron force-time cooling curve than unmodified starch under the same conditions. This reconstituted starch (or saponified starch ester) is most preferably formed from a starch ester having a saponifiable degree of substitution of at least 0.015.
Although it has not yet been determined precisely how this starch material functions, it has been shown to have the ability to permit machine speeds that are ordinarily at least about 50% or 100% higher than have previously been obtained utilizing conventional starch materials such as unmodified starch.
This property of permitting higher operating speeds (in excess of 180 , more preferably in excess of 230, meters per minute) is obviously of substantial importance. It permits substantial increases in productivity, and does so with virtually no additional capital expense.
Unfortunately, the foregoing drawback of poor rheological characteristics common in the no-carrier adhesives can greatly limit these increases in operational machine speeds. As a consequence, the improvement of the present invention is particularly useful with this special class of improved, starch ester-derived adhesives.
The instant invention will be better understood by the following detailed discussion of several specific embodiments. In this discussion, all parts and percentages are by weight, on a commercial basis, unless expressly stated to be otherwise. The commercial basis for the starch materials includes about 12% moisture by weight.